In recent years, wireless communication technology has had significant progress in the fields of broadcasting and communication and has overcome reliability-related problems such as momentary disconnections specific to wireless communication. Accordingly, wireless communication technology has increasingly found its way into the fields of control and measurement in which higher reliability is required than in the fields of broadcasting and communication.
Particularly in the fields of control and measurement, the equipment making up social infrastructure (called the social infrastructure equipment hereunder) is required to ensure higher reliability of communication quality and of communication hardware (i.e., longer service life) than general commercial equipment in the fields of broadcasting and communication. The social infrastructure equipments are, for example, an elevator system shown in FIG. 14 and a transformation installation monitoring system in FIG. 15.
Compared with general commercial equipment, the social infrastructure equipment is overwhelmingly large in size and is built sturdily using metallic members. The social infrastructure equipment itself can a scatter electromagnetic waves. Thus the wireless communication amid the social infrastructure equipment often carried out in an environment in which multiple waves (multiple passes) generated due to scattering interfere with one another. For this reason, it has been desired to provide a highly reliable wireless communication even in the environment where multiple-wave (multi-pass) interference occurs.
A plurality of electromagnetic waves have their electromagnetic wave energies cancelled out through interference therebetween when a difference in the distance between a transmitting point and a receiving point of the waves is an odd multiple of a half wavelength of another wave, which makes communication impossible. Conventionally, this problem was circumvented using a spatial diversity technique whereby a plurality of antennas are installed a half wavelength apart in space. The spatial diversity technique involves setting up multiple antennas so that when the electromagnetic wave energy received by one antenna is cancelled, the electromagnetic wave energy received by another antenna a half wavelength apart is increased by interference, whereby either of antennas can accomplish successful reception.
In the social infrastructure equipment, the electromagnetic wave generated by a wireless transmitter is reflected by the social infrastructure equipment itself to become multiple waves (multiple passes) that can reach a receiver in all directions. Thus when the spatial diversity technique is used, numerous antennas are needed. For example, even if it is assumed that multiple waves (multiple passes) are restricted to arriving in a planar direction, a plurality of arranged antennas need to be prepared. Since the distance between adjacent antennas is a half wavelength of the electromagnetic wave to be received, the dimensions of the antennas can exceed the size of an installation acceptable for the social infrastructure equipment.
The abstract and FIG. 3 of Patent document 1 (JP-10(1998)-135919-A) disclose a technique for rotating a radio wave polarization plane to suppress the effects of fading and noise in wireless communication. Furthermore, paragraph 0006 in the specification of Patent Document 1 discloses that “there are at least provided two pairs of dipole antennas positioned perpendicular to each other and extending at right angle in the transmitting direction so as to rotate a radio wave polarization plane for transmission, and a transmitter having two sets of balanced modulated wave output to excite the antennas on the transmitting side; and a receiver for detecting and receiving the rotating polarization plane of the incoming radio wave on the receiving side.”
In Patent Document 2 (JP-2(1990)-291731-A), on page 184, from line 13 in the top right block to line 2 in the bottom left block, a technique is disclosed for rotating the radio wave polarization plane so as to eliminate the effects of fading in wireless communication and thereby to transmit and receive high-quality signals. An application example 6 described in Patent Literature 2, on page 192, from line 13 in the top left block to line 12 in the top right block, discloses a technique for adding or subtracting the frequency of polarization plane revolutions to or from the carrier frequency, thereby achieving long-wavelength transmission using small (short) antennas.